The invention relates to an apparatus and method for weld cladding of metallic cylindrical workpieces and, in particular, to the weld cladding of the cylindrical fluid flow conduit of a valve body. The conduit is held at an angle with respect to the normal horizontal and vertical planes during the welding process to improve weld bead configuration.
As is known, it is often necessary to plate or clad the interior or exterior surfaces of metal components to improve the wearability of such components or to improve the corrosion resistant properties thereof. Commonly, a welding apparatus is utilized to provide the plating or cladding by depositing a weld material on the surface of the workpiece such as a valve body and then machining the welded area to the proper shape to provide a smooth surface to improve wearability or corrosion resistant characteristics of the fluid flow pathway.
There are several different types of welding processes each with its own set of advantages and disadvantages. The present invention utilizes a gas metal arc welding (GMAW) technique or actually a variation thereof called short circuiting arc welding (short arc). GMAW is an electric arc welding process which produces coalescence of metals by heating them with an arc established between a continuous filler metal electrode (consumable) and the workpiece. Shielding of the arc and molten weld bead is obtained entirely from an externally supplied gas or gas mixture. GMAW short arc welding uses the lowest range of welding current and electrode diameters associated with GMAW.
In utilizing the short arc GMAW technique it is important to control the dilution percentage of the weld bead to base metal which is defined as the amount of filler material penetrating into the base metal material divided by the total amount of filler material deposited on the surface to be welded. As the dilution percentage increases the amount of base metal imperfections drawn into the weld bead increases which can destroy the effectiveness of the corrosion resistant layer of weld material.
Prior art apparatus and methods utilizing short arc welding and GMAW techniques are not particularly new in the industry. Each of U.S. Pat. Nos. 4,215,809, issued Aug. 5, 1980 to Davis, and 4,242,981, issued Jan. 6, 1981 to Bernard et al, show an apparatus utilizing such welding processes. The prior art apparatus includes a mandrel adapted for supporting a cylindrical workpiece for rotation about its longitudinal axis. A wire-fed welding torch is supported adjacent to the interior surface of the workpiece such that as it is rotated by the mandrel welding material is deposited circumferentially about the interior surface thereof. The torch is supported for longitudinal movement with respect to the workpiece in response to the rotation thereof until the welding material is deposited over the entire interior surface. However, with these apparatus it is difficult to control the weld bead tie-ins which is the manner in which one bead fuses with its adjacent weld bead and oftentimes gaps appear between weld beads thereby destroying the effectiveness of the corrosion resistant cladding. The dilution levels between the weld bead and base material is also hard to control and, therefore, it is frequently necessary to weld two layers of cladding, one on top of the other, over the entire surface to be welded to obtain a solid machinable layer of weld material.